1. Field of the Invention
The present invention relates to an image forming apparatus including a fixing device which separates sheets of recording media by injecting compressed air.
2. Description of the Background
In electrophotographic image forming apparatuses such as printers and copiers, a toner image is formed on a recording medium (e.g., a sheet of paper) and fixed thereon by application of heat and pressure from a fixing device. The fixing device generally includes a heating member (e.g., a heating roller) and a pressing member (e.g., a pressing roller). The pressing member is pressed against the heating member to form a fixing nip through which the sheet having the toner image passes to receive heat and pressure. As the heating member, seamless belts are widely used. Such seamless-belt-like fixing members (hereinafter “fixing belts”) contribute to energy saving because their heat capacity are so small that the warm-up time can be reduced.
The toner image fused on a sheet contacts the fixing roller or belt. Thus, the fixing roller or belt is generally coated with a fluororesin having high releasability. Additionally, a separation claw is generally provided for separating the sheet from the fixing roller or belt. Disadvantageously, the separation claw is likely to make scratches on the fixing roller or belt, which appear as undesired lines in the resulting image.
Typical monochrome image forming apparatuses employ a metallic roller whose surface is covered with TEFLON (trademark) as the fixing roller. Such a roller is resistant to scratch making by the separation claw and can be used for an extended period of time.
On the other hand, typical full-color image forming apparatuses employ a member whose surface is covered with a silicone rubber coated with a fluorine-based material (e.g., a PFA tube having a thickness of several tens micron) or a silicone oil, for the purpose of improving chromogenic properties of the resulting image. Such a configuration makes the surface of the fixing member too soft to be resistant to scratch making. Scratches made on the surface result in undesired lines in the fixed image. Thus, recent full-color image forming apparatuses do not employ a member that contacts the fixing member, such as the separation claw, and separate sheets from the fixing member without any member.
In such sheet separation without any member, when toner is adhesive to the fixing member, a sheet having the fixed toner image is likely to wind around the fixing member, causing paper winding jam. In particular, a full-color toner image, comprised of multiple toner layers, is more likely to cause paper winding jam because of its higher adhesiveness to the fixing member.
The following sheet separation systems are widely employed recently.    1) A non-contact separation plate system in which a separation plate is provided parallel to the longitudinal and width direction of a fixing roller or belt, forming a micro gap (about 0.2 to 1.0 mm) between the fixing roller or belt.    2) A non-contact separation claw system in which multiple separation claws are provided at a predetermined interval, forming a micro gap (about 0.2 to 1.0 mm) between a fixing roller or belt.    3) A self-stripping system in which sheets self-separate from a fixing roller or belt due to flexibility of the sheets and elasticity of flexure of the fixing roller or belt.
Additionally, Japanese Patent Application Publication No. (hereinafter “JP-A”) 2009-31759 also describes a separation plate system.
In the above systems, a gap between an exit of the fixing nip and a sheet guide plate is relatively large. Therefore, thin sheets, sheets having little margin on their leading edge, or sheets having solid images (e.g., photograph) are likely to pass through the gap without separating from the fixing member, causing paper winding jam. Or such sheets are likely to bump the separation plate or claw, causing paper jam.
In view of such situations, there have been proposals to separate sheets from a fixing member by air injection. For example, JP-S51-104350-A, Japanese Patent No. 2876127, JP-H11-334191-A, JP-2007-187715-A, JP-2007-240920-A, JP-2008-102408-A, and JP-2007-86132-A propose to inject air into between a fixing member and a sheet having the fixed toner image so that the sheet forcibly separates from the fixing member.
For the purpose of saving energy and downsizing compressor (i.e., compressed air generator), generally, compressed air is injected for only a part of the time period during which a leading edge of a sheet passes the fixing nip, so that only the leading edge separates from the fixing member and is introduced into a separation plate or sheet guide.
On the other hand, full-color toner images generally include toner in a large amount and are high at gloss. A sheet having such a full-color toner image separates from the fixing member at immediately downstream from the exit of the fixing nip by the action of the injected compressed air, but starts to wind around the fixing member after stopping injecting compressed air while being introduced into a separation plate or sheet guide.
As a result, every portion on the fixing member receives different amounts of heat depending on whether or not compressed air is injected. Thus, the resulting image has uneven surface smoothness, i.e., uneven gloss and image density.
FIG. 1A schematically illustrates a behavior of a sheet P when compressed air is injected. In FIG. 1A, the sheet P is separated from a fixing belt at immediately downstream from the exit of the fixing nip. FIG. 1B schematically illustrates a behavior of a sheet P when compressed air is not injected. In FIG. 1B, the sheet P is separated from a fixing belt while slightly winding to the fixing member. FIG. 2 schematically illustrates a solid image showing that the gloss level is different between areas corresponding to portions on the fixing member where compressed air is injected or not. Such gloss difference is generated due to the difference in received heat quantity between the portions on the fixing member where compressed air is injected or not.
The problem of uneven gloss and image density can be solved by injecting compressed air over an entire period of time during which an entire sheet, from a leading edge to a rear edge, passes through the fixing nip.
To separate a leading edge of a sheet from the fixing member, compressed air is required to be injected at a high pressure. This is because the leading edge tends to wind around the fixing member. In particular, the separation force is required to exceed the total of the adhesive force between the fixing member and toner, and flexibility of the sheet. The injection pressure required at this time is hereinafter referred to as the pressure Ps.
After the leading edge is separated from the fixing member and introduced into a sheet guide, the flexibility of the sheet now acts as separation force. Thus, the injection pressure needs not to be high at this time. The injection pressure required at this time is hereinafter referred to as the pressure Pk.
In summary, when separating a leading edge of a sheet from a fixing member, at least the pressure Ps is required. After separating the leading edge of the sheet from the fixing member and introducing it into a sheet guide, at least the pressure Pk is required.
In continuous sheet feeding, compressed air is also continuously injected while reducing the tank pressure of a compressor. To keep injecting compressed air at the required injection pressure Ps even when the tank pressure is reduced, flow rate (per unit time) is required to be as large as possible. As a result, the compressor gets larger undesirably.